Electrophotographic printing processes, sometimes termed electrostatic printing processes, typically involve creating an image on a photoconductive surface, applying an ink having charged particles to the photoconductive surface, such that they selectively bind to the image, and then transferring the charged particles in the form of the image to a print substrate.
The photoconductive surface may be on a cylinder and is often termed a photo imaging plate (PIP). The photoconductive surface is selectively charged with a latent electrostatic image having image and background areas with different potentials. For example, a liquid electrophotographic (LEP) ink composition including ink particles in a liquid carrier can be charged by applying a developing voltage to the LEP ink composition to provide charged ink particles which are then brought into contact with the selectively charged photoconductive surface. The charged ink particles adhere to the image areas of the latent image while the background areas remain clean. The image is then transferred to a print substrate (e.g. paper) directly or, by being first transferred to an intermediate transfer member, which can be a soft swelling blanket, which is often heated to fuse the solid image and evaporate the liquid carrier, and then to the print substrate.
Some previous LEP ink compositions comprising cyan, magenta or black colorants have been found to suffer from electrical fatigue. Electrical fatigue may cause the charging property of a LEP ink composition to change, for example an increase in particle conductivity, when exposed to electrical fields for prolonged periods of time. If particle conductivity of the LEP ink composition changes, the number of particles transferred to the photoconductive surface in a liquid electrostatic printing process changes for a given developing voltage, resulting in a different thickness of ink being transferred to the print substrate which may cause a decline in the optical density of the printed image. Some previous solutions to this problem include correcting the developing voltage applied to the LEP ink composition in order to preserve the optical density.